The invention is in the field of fluorescent lamps of the rapid start type, the lamps being provided with thermal switches, responsive to cathode heat, for turning off the cathode heating current after starting and during lamp operation.
Rapid start fluorescent lamps are provided with cathode heating current, for heating the cathodes to electron-emitting temperature so that the lamps start quickly without damaging the electron-emitting material of the cathodes. The cathode heating consumes about one and one-half to two watts of electrical power per cathode. While the lamps are operating, "hot spots" form on the cathodes and can provide adequate electron emission without the need for continuing to supply heating current through the cathodes. Thus, turning off the cathode heating current when the lamps are operating can save about three or four watts of electrical energy per lamp, resulting in considerable energy and money savings in lighting systems, such as in large buildings, having hundreds or thousands of lamps.
U.S. Pat. Nos. 4,097,779 and 4,114,968 to Latassa disclose rapid start fluorescent lamps provided with a thermal cutout switch near each cathode, and in electrical series with the associated cathode, for turning off the cathode current after the lamps start and while they are operating. More specifically, these patents disclose U-shaped bimetal switches sealed in glass envelopes and mounted near each cathode. After each cathode is heated sufficiently by the heating current (in a second or so), heat from the cathode causes the nearby bimetal switch member to bend and open the current circuit to the cathode.
The manufacture of fluorescent lamps involves coating the tungsten cathode coils with an electron emission coating, such as a mixture of alkaline earth oxides in the form of carbonates. After the lamps are assembled, with a cathode mount in each end region of a glass tube, the cathodes are "activated" by passing current through them to heat them, for a sufficient time such as 20 seconds, to convert the carbonates into oxides. However, with the aforesaid cathode current cutout switches in the lamps, these switches when suitably designed for reliably functioning as described above, will turn off the activation cathode heating current prior to complete activation of the cathodes, thus preventing proper activation. The aforesaid U.S. Pat. No. 4,114,968 solves this problem by connecting fuse wires across the thermal switches, for shorting the switches and permitting proper activation of the cathodes. The fuses are then "blown", i.e., severed, by applying an electrical pulse through each of the series-connected fuses and cathodes. The fuses can be a difficult and critical item since they must be able to carry the cathode activation current and also be capable of being "blown" by a current pulse of insufficient strength to damage the cathode. Also, the timing is critical, since the fuse-blowing pulse must be applied while the thermal switch is in open condition so it will not short-circuit the pulse away from the fuse.